Marine vessels have a wide variety uses for transportation of people and cargo across bodies of water. These uses include fishing, military and recreational activities. Marine vessels may move on the water surface as surface ships do, as well as move beneath the water surface, as submarines do. Some marine vessels use propulsion and control systems.
Various forms of propulsion have been used to propel marine vessels over or through the water. One type of propulsion system comprises a prime mover, such as an engine or a turbine, which converts energy into a rotation that is transferred to one or more propellers having blades in contact with the surrounding water. The rotational energy in a propeller is transferred by contoured surfaces of the propeller blades into a force or “thrust” which propels the marine vessel. As the propeller blades push water in one direction, thrust and vessel motion are generated in the opposite direction. Many shapes and geometries for propeller-type propulsion systems are known.
Other marine vessel propulsion systems utilize water jet propulsion to achieve similar results. Such devices include a pump, a water intake or suction port and an exit or discharge port, which generate a water jet stream that propels the marine vessel. The water jet stream may be deflected using one or more “deflectors” to provide marine vessel control by redirecting some water jet stream thrust in a suitable direction and in a suitable amount. For example, the water jet stream may be deflected using a reversing deflector (e.g., a reversing bucket), a steering deflector (e.g., a steering nozzle), and/or any other suitable type of deflector.
It is sometimes more convenient and efficient to construct a marine vessel propulsion system such that the net thrust generated by the propulsion system is always in the forward direction. The “forward” direction or “ahead” direction is along a vector pointing from the stern, or aft end of the vessel, to its bow, or front end of the vessel. By contrast, the “reverse”, “astern” or “backing” directing is along a vector pointing in the opposite direction (or 180 degrees away) from the forward direction. The axis defined by a straight line connecting a vessel's bow to its stern is referred to herein as the “major axis” of the vessel. A vessel has only one major axis. Any axis perpendicular to the major axis is referred to herein as a “minor axis.” A vessel has a plurality of minor axes, lying in a plane perpendicular to the major axis. Some marine vessels have propulsion systems which primarily provide thrust along the vessel's major axis, in the forward or backward directions. Other thrust directions, along the minor axes, are generated with awkward or inefficient auxiliary control surfaces, rudders, planes, deflectors, etc. Rather than reversing the direction of a ship's propeller or water jet streams, it may be advantageous to have the propulsion system remain engaged in the forward direction while providing other mechanisms for redirecting the water flow to provide the desired maneuvers.
A typical capability of marine vessels is the ability to steer the vessel from side to side. Some systems, commonly used with propeller-driven vessels, employ “rudders” for this purpose. A rudder is generally a planar water deflector or control surface, placed vertically into the water, and parallel to a direction of motion, such that left-to-right deflection of the rudder, and a corresponding deflection of a flow of water over the rudder, provides steering for the marine vessel.
Other systems for steering marine vessels, commonly used in water jet stream propelled vessels, rotate the exit or discharge nozzle of the water jet stream from one side to another. Such a nozzle is sometimes referred to as a “steering nozzle,” which is one example of a steering deflector. Hydraulic actuators may be used to rotate an articulated steering nozzle so that the aft end of the marine vessel experiences a sideways thrust in addition to any forward or backing force of the water jet stream. The reaction of the marine vessel to the side-to-side movement of the steering nozzle will be in accordance with the laws of motion and conservation of momentum principles, and will depend on the dynamics of the marine vessel design.
A primary reason why waterjet powered craft are extremely efficient at high speeds is the lack of appendages located bellow the waterline. Typical appendages that can be found on non-waterjet driven craft (i.e., propeller driven) are rudders, propeller shafts, and propeller struts. These appendages can develop significant resistance, particularly at high speeds.
The lack of appendages on waterjet driven craft also provides a significant advantage in shallow water, as these craft typically have much shallower draught and are less susceptible to damage when run aground, as compared to craft with propellers bellow the hull.
Notwithstanding the negative effects on craft resistance, some appendages are of considerable value with respect to other craft dynamic characteristics. Although a significant source of drag at high speeds, a rudder is a primary contributor to craft stability when moving forward through the water, particularly when traveling at slow to medium speeds.
In simple terms, a rudder is a foil with a variable angle of attack. Actively varying the angle of attack (e.g., a turning maneuver) will increase the hydrodynamic force on one side of the rudder and decrease the hydrodynamic force on the opposite side, thereby developing a net force with a transverse component to yaw the craft in the desired direction.
Referring to FIG. 1 many craft are equipped with lifting devices known as trim-tabs (also known as tabs or transom-flaps) 200 or interceptors 206 (see FIG. 2). A trim tab 200 can be thought of as a variable-angle wedge that mounts to the transom 203 of a vessel that, when engaged with a water stream, creates upward force 204 on both the trim tab 200 and the hull bottom 205. Varying the actuator 201 position will create varying amounts of hydrodynamic force 204 on the vessel. For example, extending the actuator 201 so as to actuate the trim tab further into the water stream will increase the angle of attack of the wedge, thereby increasing the hydrodynamic force 204 on the vessel. In contrast, referring to FIG. 2, an interceptor 206, mounted to transom 203 of a vessel and actuated by actuator 207, intercepts the flow of water under the transom of the vessel with a small blade 206 and creates an upward hydrodynamic force on the hull bottom 205. These devices that are found in both propeller and waterjet driven craft can be actuated to develop a hydrodynamic lifting force at the transom (stern) to trim the bow down, assisting the craft in getting up on plane and adjust the heel angle of the craft. Both trim-tabs and interceptors typically develop forces in the opposite direction of the actuation and along the same plane as the control surface motion.
It should be understood that while particular control surfaces are primarily designed to provide force or motion in a particular direction, these surfaces often also provide forces in other directions that may not be desired. For example, a steering deflector such as a steering nozzle, which is primarily intended to develop a yawing moment on the craft, in many cases may develop a rolling or heeling effect. This is due to the relative orientation of the nozzle's turning axis. Referring, for illustration purposes, to FIGS. 3A, 3B, it is to be appreciated that in many waterjet propelled craft, the rotational axis of the steering nozzle 312, 314 is orthogonal to the bottom surface 16, 18 of the craft such that the rotational (transverse) thrust component generated by the steering nozzle is applied in a direction parallel to the bottom surface of the craft. Because of, for example the V-shaped or deep V-shaped hull, a rotational thrust component is generated at an angle (with respect to a horizontal surface) close or equal to the dead rise angle of the hull at the transom, which thereby causes a rolling or heeling moment in addition to a yawing (rotational) moment. The net rolling/heeling force imposed on a dual waterjet propelled craft can be equal to twice the force developed by a single waterjet. This is because the nozzles are typically controlled in unison when a waterjet driven craft is in a forward cruising or transiting mode.
Similarly, trim-tabs and interceptors 320, 322 are generally mounted at the transom 324, close to the free surface of the water such that a trimming force is developed orthogonal or perpendicular to the bottom surface 316, 318 of the hull at the transom. While the purpose of the trim tabs and interceptors is to develop up/down trimming forces at the transom, an inward component is also developed because a force is developed at an angle (with respect to a horizontal surface) close or equal to the dead rise angle of the hull at the transom plus 90 degrees. When both trim-tabs or interceptors are actuated together, the side components cancel out and the net force is close to or exactly vertical. When one tab or interceptor is actuated more than the other, for example when a rolling or healing force is desired, a side or yawing component is developed, causing a turning effect as well. The relative magnitude of the yawing component increases with increased dead rise angle. FIG. 4A illustrates how actuating the interceptor or trim-tab differentially in order to create a rolling force may also induce an unwanted yaw force. FIG. 4B illustrates how actuating the steering nozzles in order to create a yawing force may also induce an unwanted roll moment. These unwanted yawing and rolling forces in planning craft can make it difficult to control the craft at high speeds, particularly when automatic controls systems are employed such as Autopilots for automatically controlling the vessel heading and Ride Control Systems for minimizing pitch and roll disturbances.